Fix AST print of first class callable in anon class expr

This will cause a compile error later, but the expression is
printed first.

Fixes oss-fuzz #37473.

Zend/tests/first_class_callable_assert3.phpt

@@ -0,0 +1,10 @@
+--TEST--
+new class(...)  in assert
+--FILE--
+<?php
+
+assert(new class(...) {});
+
+?>
+--EXPECTF--
+Fatal error: Cannot create Closure for new expression in %s on line %d

Zend/zend_ast.c

@@ -2036,7 +2036,8 @@ static ZEND_COLD void zend_ast_export_ex(smart_str *str, zend_ast *ast, int prio
 					zend_ast_export_attributes(str, decl->child[3], indent, 0);
 				}
 				smart_str_appends(str, "class");
-				if (zend_ast_get_list(ast->child[1])->children) {
+				if (!zend_ast_is_list(ast->child[1])
+						|| zend_ast_get_list(ast->child[1])->children) {
 					smart_str_appendc(str, '(');
 					zend_ast_export_ex(str, ast->child[1], 0, indent);
 					smart_str_appendc(str, ')');